The broken steel rings were examined in the TSB Engineering Laboratory. The examination showed that both rings had ruptured under overload at the welded joint. It was determined that one ring had ruptured at the time of the occurrence, and that there was no prior cracking. The other had been cracked before the occurrence. Analysis of the welds showed that they did not penetrate the entire cross-section and that they had not been heat-treated after welding. The rings therefore did not have their maximum strength of 74,000lbs. The stronger ring could bear a load of about 30,000lbs, whereas a similar ring without a weld joint could bear a load of about 1,000lbs.Analysis The broken steel rings were examined in the TSB Engineering Laboratory. The examination showed that both rings had ruptured under overload at the welded joint. It was determined that one ring had ruptured at the time of the occurrence, and that there was no prior cracking. The other had been cracked before the occurrence. Analysis of the welds showed that they did not penetrate the entire cross-section and that they had not been heat-treated after welding. The rings therefore did not have their maximum strength of 74,000lbs. The stronger ring could bear a load of about 30,000lbs, whereas a similar ring without a weld joint could bear a load of about 1,000lbs. At the time of departure, only moderately rough seas were expected. The truck parked at the forward end of the vessel was secured to the ferry deck with only two tie-downs. An exceptionally large wave made the vessel pitch and then roll violently. Under the effect of the pitching and rolling, the truck broke its tie-downs and then overturned. One of the steel rings used to secure the truck to the deck was cracked before the occurrence. The welding for the joints in both rings did not penetrate the full cross-section.Findings At the time of departure, only moderately rough seas were expected. The truck parked at the forward end of the vessel was secured to the ferry deck with only two tie-downs. An exceptionally large wave made the vessel pitch and then roll violently. Under the effect of the pitching and rolling, the truck broke its tie-downs and then overturned. One of the steel rings used to secure the truck to the deck was cracked before the occurrence. The welding for the joints in both rings did not penetrate the full cross-section. A truck being transported on the forward deck of the ferry L'HRITAGEI broke its tie-downs and overturned because there were not enough tie-downs securing it and the vessel met with rougher seas than expected. The steel rings used to secure the tie-downs to the deck were in poor condition and could not withstand the strain.Causes and Contributing Factors A truck being transported on the forward deck of the ferry L'HRITAGEI broke its tie-downs and overturned because there were not enough tie-downs securing it and the vessel met with rougher seas than expected. The steel rings used to secure the tie-downs to the deck were in poor condition and could not withstand the strain. Following the occurrence, corrective measures were taken to prevent vehicles transported on the ferry (weighing as much as 20 to 25tonnes) from shifting and turning over. All securing rings attached to the main deck were replaced with rings capable of supporting a test load of 15,000lbs. From now on, all trucks and heavy vehicles will be secured with at least four tie-downs on each side.Safety Action Taken Following the occurrence, corrective measures were taken to prevent vehicles transported on the ferry (weighing as much as 20 to 25tonnes) from shifting and turning over. All securing rings attached to the main deck were replaced with rings capable of supporting a test load of 15,000lbs. From now on, all trucks and heavy vehicles will be secured with at least four tie-downs on each side.